High stretch paper and method of producing the same

ABSTRACT

A method for manufacturing an extensible paper, is herein described. The method comprises the steps of: providing pulp fibers in form of a pulp having a first length-weighted average curl index; mechanically treating the pulp fibers using a pulp compression process to induce fiber deformations such that the resulting pulp fibers have a second length-weighted average curl index higher than the first length-weighted average curl index; forming a wet web using the mechanically treated pulp fibers; drying the wet web under restraint to form a dried web; and adding a polymer to the dried web. An extensible paper is also described that comprises: pulp fibers having a length-weighted average curl index C LW ; and a polymer in an amount of from 2 to 40 wt % based on the weight of the extensible paper; wherein the extensible paper has a Gurley air resistance below 20 s/100 mL and an elongation at break of at least 7%.

CROSS-REFERENCE TO RELATED APPLICATIONS

The present application claims benefit of U.S. Provisional Application No. 62/624,879 filed Feb. 1, 2018, the content of which is hereby incorporated by reference in their entirety.

FIELD

The present relates to papers with high extensibility, their composition and method for producing the same. More particularly, the present relates to high stretch paper including some pulp fibers that have been treated mechanically in order to induce fiber curl and kinks.

BACKGROUND

The extensibility of paper, also referred to as stretch or elongation at break, is a key property of paper. Paper products are generally brittle and most of them have low extensibility. As a result, paper is rarely used in applications involving large deformations of the material. Many different approaches have been tested over the years to increase the stretch of paper and overcome this limitation.

Papermaking treatment to increase stretch potential

One of the most important factors controlling the extensibility of paper is the amount of sheet shrinkage taking place during drying. The underlying mechanism is the shrinkage of fibers in the transverse dimensions as water evaporates from the cell wall. That process takes place after fiber-fiber bonds have formed in the network so that fibers cannot move relative to each other. As a result, transverse shrinkage of one fiber compresses its crossing fiber in the axial direction, and the paper shrinks in all directions. The amount of shrinkage incurred by the paper depends very strongly on the restraint applied during drying: sheets dried without restraint shrink a lot more than those dried fully restrained. The amount of drying shrinkage that can take place under unrestrained conditions is limited to between 3 and 10%. Furthermore, while free-drying of paper is a simple method to increase stretch, its applicability to commercial products manufactured on a paper machine is limited.

Stretch of paper can also be increased by inducing deformations of the fibrous network. This is achieved by applying in-plane compressive forces on the wet paper during papermaking. An example of that approach is the Clupak process which is described in U.S. Pat. 2,624,245. In that process, the wet sheet is pressed between two rolls rotating at different speeds. One is a steel roll while the other is covered with a rubber that is stretched in front of the nip. The sheet adheres to the rubber surface as it enters the nip and gets compacted in the machine direction (MD) as the rubber is allowed to contract. The amount of compaction can be substantial and is controlled by adjusting the speed difference between the two rolls. The Clupak and other related processes (e.g., Expanda, see also U.S. Pat. No. 7,918,966) are commonly used in the manufacturing of sack grade paper. They can provide gains in MD stretch as high as 30%. However, they have little impact on the elongation at break in the cross direction (CD). Furthermore, only a small fraction of existing paper machines are fitted with the equipment needed to produce such papers.

The creping process is another very effective approach to increase stretch in the machine direction. It is commonly used in the production of tissue paper and is done by a doctor blade that scrapes the paper off a drying cylinder (Yankee). The high-speed collision between the paper and blade results in large out-of-plane deformations of the paper in the machine direction. The length scales associated with these deformations depend on a number of factors including pulp characteristics, blade geometry, strength of adhesion between paper and Yankee and speed difference between the Yankee and the final section of the paper machine. After creping, the stretch of paper in the machine direction is typically above 10%. However, creping, like the Clupak process, has little impact on the elongation at break in the CD direction. This severely limits the range of applications for which creped papers can be used.

Fiber selection

It is also possible to increase sheet extensibility by selecting pulp fibers with desirable characteristics or by modifying these fibers through mechanical, chemical or combined mechanical/chemical means. The mechanical properties of wood pulp fibers are determined to a large extent by the thickest layer (S2) of the fiber cell wall. That layer consists of helically wound cellulose fibrils oriented at some angle relative to the fiber axis. The fibrils are held together by some hemicellulose and lignin. In general, the larger the fibril angle, the more extensible the fiber is. For instance, juvenile fibers, which have high fibril angle, tend to be more extensible than latewood fibers. The impact of fibril angle on stretch has been observed in both softwood and hardwood unbleached kraft pulps. Gains in paper stretch of at most 2 to 3% can be achieved by utilizing pulp fractions with high fibril angle.

Chemical addition

The addition of extensible polymers to paper is another approach for increasing its elongation at break. The objective is to produce a composite material with properties intermediate between those of the polymer and those of the paper produced without it. Polymers available as aqueous dispersions (latexes) are commonly used for that purpose. They are generally added to the pulp prior to papermaking or to the pre-formed fiber network during papermaking. For example, Alince (1977, Svensk Papperstidning, 13: 417) has shown that addition of styrene-butadiene latex to a kraft pulp could increase paper stretch by up to 4%. Ankerfors and Lindström (WO 2011/087438) similarly added various amounts of thermoplastic latex to a never-dried bleached softwood kraft pulp to produce a mouldable material. Handsheets prepared with 17% by weight of polylactide latex and warm-pressed under varying conditions of temperature and pressure had elongations at break around 9%. Waterhouse (1976, Tappi J., 59: 106) used the second approach: he impregnated wet (never-dried) handsheets with latex and found that, after wet pressing and drying, the elongation at maximum load of the papers could be as high as 18%. However, these values of elongation at maximum load were observed at levels of latex addition above 30% and with minimal restraint applied to the sheet during drying, a situation not representative of typical conditions on a paper machine. Stokes (U.S. Pat. No. 4,849,278) similarly prepared a flexible and stretchable paper by saturating a web that was previously creped while in a semi-dry state with a soft polymer characterized by a glass transition temperature between −45° C. and 0° C. The resulting saturated substrate, which has an elongation at break in machine direction in the range from about 18% to 30%, can be used as label stock for squeezable containers. The elongations at break obtained in the CD direction were significantly lower as the approach relies on the use of a creping process.

Foam-forming

Torniainan et al. (U.S. 20170260694) used a foam-forming process to produce extensible fiber sheets comprising natural and reinforcing fibers as well as a binder and a foaming agent. Sheets produced with their approach have elongations at break between 3 and 50%. Depending on furnish and choice of binder, they can also be biodegradable and/or recyclable. However, the fiber sheets are produced with a foam-laid method which is significantly different from conventional papermaking. As a result, extensive and costly modifications would be required in order to produce such extensible fiber sheets on a conventional paper machine.

Fiber chemical treatment to increase stretch potential

A number of different chemical processes have been used to modify pulp fibers in order to increase paper stretch. For example, hydroxyethylation and hydroxypropylation have been found to have a positive impact on both paper strength and stretch. The selective oxidation of some hydroxyl groups of cellulose has also been shown to improve both single fiber and network extensibility. Chemical grafting of acrylate polymers onto the pulp fibers is another modification that can improve elongation at break. In general, chemical modification of the fiber involves a decrease in crystallinity of the cellulose. In turn, this leads to an increase in fiber swelling, drying shrinkage and/or the amount, strength or compliance of fiber-fiber bonds in the sheet; all of these effects generally have a positive impact on sheet elongation. That impact depends strongly on the specific process used as well as the conditions under which the paper is dried. Regarding such chemical processes, the highest elongation at break measured was slightly below 16% on hydroxypropylated sheets dried unrestrained. The same sheets dried under more realistic restrained conditions had an elongation at break of only 5%. It should also be noted that the approaches discussed above involve chemical reactions that can be costly and difficult to scale-up.

Fiber treatment to induce fiber curl and kink

In the pulping process, wood chips are subjected to various treatments including mechanical, chemical or both to produce individualized fibers with suitable properties for papermaking applications. The mechanical treatment causes some fiber structural distortions such as curl and kinks. Those distortions can cause loss in fiber properties such as bonding potential, and ultimately lead to loss in paper strength properties. With appropriate papermaking processes, most of those distortions can be removed; however, for some specialty grades such as tissue and towel, the presence of those structural features results in some desirable properties for those grades such as high extensibility, higher bulk, softness and absorbency. Therefore for those applications, pulp manufacturers prefer to preserve fiber curl and kinks and in some cases, even further increase and enhance those structural features.

Curl and kinks are produced when wood fibers are subjected to continuous application of normal/compression and shear forces, possibly followed by chemical cross-linking. Several technologies have been either used, or in some cases developed, to preferentially induce high level of curl and kinks in mechanical and/or chemical wood fibers. Depending on the equipment and processes, those technologies can be grouped into the following main categories: (i) mechanical treatment, (ii) chemical cross-linking, (iii) heat treatment or, (iv) a combination thereof.

Mechanical treatment to induce fiber curl and kink

The stretch potential of papermaking fibers depends strongly on the amount of axial compression they undergo during production. Specifically, fibers in the wood or pulp experience mechanical stresses in operations such as chipping, defiberizing, plug-screw feeding and refining. For pulps, the amount of fiber deformation typically increases with the consistency of the suspension. As a result, various medium or high consistency mechanical treatments can be used to induce dislocations and microcompressions in pulp fibers. While these treatments increase the stretch potential of the fibers, papers made from these fibers are usually weak and they do not exhibit improved stretch properties.

The decrease in tensile strength is attributed to large-scale fiber deformations such as kinks and curl produced during the mechanical treatment. These deformations lead to poor bonding between the fibers and to non-uniform stress distribution in the network when the paper is strained. As a result, the sheet breaks before the full stretch potential of the fibers can be realized. One mechanical treatment that can improve stretch is high-consistency refining (HCR) of the pulp under atmospheric or pressurized conditions. During HCR, fibers undergo continuous normal and shear fatigue loading causing structural distortions and their outer cell wall is also fibrillated, thus improving fiber-fiber bonding. As a result, papers made from refined fibers are both stronger and more extensible than those produced from the unrefined pulp. Paper stretch can also be improved by combining high and low-consistency refining of the pulp. The low-consistency refining is performed in order to further straighten the pulp fibers after the high-consistency treatment. It promotes fiber-fiber bonding and improves the ability of the fiber network to transfer stress when placed under load. The gain in paper stretch obtained by HCR or by a combination of high-consistency and low-consistency treatments is usually limited to a few percent.

Several types of equipment have been used to apply forces inducing curl and kinks. Eber et al. (U.S. Pat. No. 4,488,932) used hammermilling and refining to subject pulp webs to shear forces to produce curly fibers. They showed that paper bulk increased when treated fibers were blended with untreated ones to produce the sheet. Kasser used a Kollergang mill to induce fiber curl (U.S. Pat. No. 4,409,065). In Kasser's process, refined kraft fibers were subjected to mechanical treatment to increase stretch for application in sack paper. Carets et al. (U.S. Pat. No. 7,390,378) used a rotating drum to subject wood fibers to forces as fibers followed a spiral downstream path inside the drum, and stated that fiber curl increased by almost 150% with this technique. Hill et al. (U.S. Pat. No. 2,516,384) proposed a conceptual design to subject wood fiber nodules to continuous compression-decompression forces to create kinks and curl. Their design consists of two off-centred concentric cylinders where the outer cylinder is stationary and the inner cylinder is rotating. The pulp nodules are in the gap between the cylinders and the rotation of the inner cylinder generates compression forces.

Steam explosion process was also used to produce curly fibers for applications such as absorbent grades. In this technique, wood fibers are cooked under high temperature and pressure. Once the required cooking time is elapsed, the cooking vessel is decompressed rapidly (to atmospheric pressure) and fibers are recovered. The sudden decompression creates structural distortions into fiber structure and produces highly curled fibers. Hu (U.S. Pat. No. 6,413,362) used a similar technique to produce curly fibers. Hu proposed mechanical treatment of fibers (in hammermill or in a refiner) prior to cooking and steam explosion. Hermans et al. (U.S. Pat. No. 5,501,768) used the commercial BIVIS equipment to induce curl into fiber structure. BIVIS is composed of a screw press where wood chips/fibers are subjected to compression-decompression zones as they move along the press flights. At various intervals, fibers are pressed and forced to pass through small holes. The continuous application of compression-decompression forces creates distortions such as curl and kinks into fiber structure.

Plug screw is another piece of commercial equipment which was used to induce fiber curl and kinks. The equipment is usually part of the pressurized refining system to create pulp plug in order to maintain pressure at the feeding port of the steaming tube. For example, Minton (U.S. Pat. No. 4,976,819) used a plug screw process and subjected fibers to compression forces varying from 2:1 to 8:1 in compression ratio. Minton produced curly fibers for tissue and multi-ply board applications.

A widely used equipment to induce fiber curl and kinks are mechanical refiners (atmospheric or pressurized), with or without plug screw. Generally the applied refining specific energy was kept low and only adequate for dispersing or defiberizing the pulp nodules. The impact of various operating conditions such as consistency, plate pattern, pressure, etc. on fiber curls and kinks was evaluated. The produced fibers were used in a wide range of applications from tissue to absorbent grades and boards.

Above results have shown that in order to achieve high fiber curl and to preserve it throughout the process, two conditions should be met; first, mechanical treatment of fibers under high pressure/temperature and second, no extra refining afterward. With high consistency refining or mechanical treatment of fibers under atmospheric condition (such as in chips impregnator), the maximum achievable curl index would generally be below 0.2 while with mechanical treatment under high pressures (such as in plug screw), curl indices of 0.2 to 0.45 could be obtained.

Mechanical and heat treatment to induce fiber curl and kink

It has been shown that when mechanical methods are used to induce curl and kinks, the structural distortions are not permanent and part of the curl is removed when fibers are agitated with water at low consistencies. It has also been shown that if after mechanical treatment, fibers are heat treated, the induced curl will stay permanent even if fibers are agitated at low consistency for an extended amount of time. A common method of heat treatment is drying fibers in flash dryers. Ko et al. (U.S. Pat. No. 6,837,970) and Hu et al. (U.S. Pat. No. 7,364,639) used mechanical treatment (in BIVIS or refiners) followed by heat treatment in flash dryers to produce permanently curly fibers. Barbe et al. (U.S. Pat. No. 4,431,479) used a similar approach, i.e. inducing fiber curl using high-consistency refining and then heat treating the curled fibers in a digester at 100° C.-170° C. and heating time of 2-60 minutes. To reduce the impact of heat treatment on mechanical pulp brightness, Barbe et al. also suggested that brightening chemicals could be added to the pulp during the heat treatment process.

Chemical treatment to induce fiber curl and kink

Chemical treatment has also been used to produce curly fibers. The chemical treatment consists of crosslinking individualized cellulose fibers (crosslinking between cellulose molecules of a single fiber rather than separate fibers) and has been thoroughly described.

Among the above-mentioned approaches to increase stretch, many can only increase stretch by a few percent, which is insufficient for some applications. Others like creping or wet compaction have a large impact on elongation, but only in the machine direction. Furthermore, those approaches require specialized equipment on the paper machine which most mills do not have access to. Chemical modification of the fibers (as described above) can be an effective way to increase stretch but it is costly and usually works better when the paper is dried without restraint.

Therefore, there is a need for a different method for producing highly stretchable paper that is cost-effective, that does not compromise the sustainable character of paper and that can be easily implemented in a mill.

SUMMARY

In accordance with the present disclosure there is provided a method for manufacturing an extensible paper, the method comprising the steps of: providing pulp fibers in form of a pulp having a first length-weighted average curl index (C_(LW1)) between 0 and 0.3; mechanically treating the pulp fibers using a pulp compression process under a pressure of from 2 to 20 bars and expending an energy of 60 to 100 kWh/tonne, to induce fiber deformations such that the resulting pulp fibers have a second length-weighted average curl index (C_(LW2)) higher than the first length-weighted average curl index and between 0.1 and 0.45; forming a wet web using the mechanically treated pulp fibers; drying the wet web under restraint to form a dried web; and adding a polymer to the dried web.

In a particular embodiment, the pulp has a consistency above 20% by dry weight before the pulp compression process.

In a particular embodiment, chemical additives are added to the pulp before it undergoes the compression process.

In a particular embodiment, the pressure of the pulp compression process is from 3 to 8 bars.

In a particular embodiment, the pulp fibers are refined prior to undergoing mechanical treatment.

In a particular embodiment, the pulp compression process is a plug screw feeding process.

In a further embodiment, the pulp compression process is a hammer mill, a Kollergang mill, a BIVIS device, an explosion pulping device or a revolving drum.

In a particular embodiment, the pulp is subjected to a separate heat treatment after the pulp compression process.

In a particular embodiment, the second length-weighted average curl index (C_(LW2)) is between 0.25 and 0.45.

In a particular embodiment, the polymer is a bio-based polymer, a biodegradable polymer and/or a petroleum-based polymer. The petroleum-based polymer can be a thermoplastic polymer or copolymer selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, acrylic polymer, acrylic copolymer, vinyl acetate polymers, vinyl acetate copolymers, and vinyl chloride-vinylidene chloride copolymers. The polymer can be in the form of an aqueous dispersion or latex.

In a particular embodiment, the polymer can be added by impregnation, and can be added in an amount of from 2 to 40 wt %. Impregnation of the dried web can comprise flooding the web with a water-based or molten polymer, removing the excess polymer from the surface of the web and drying the web under restrain to evaporate the water.

In a particular embodiment, a cross-linking agent is added to the pulp after mechanically treating the pulp fibers.

In a particular embodiment, the pulp is a mechanical, chemi-mechanical or chemical pulp.

In an embodiment, the polymer is in the form of an aqueous dispersion or latex.

In another embodiment, a chemical additive is added to the pulp prior or after mechanically treating the pulp.

In an embodiment, the chemical additive is a plasticizer.

In another embodiment, the chemical additive is a strength additive, a sizing agent, or a cross linking agent.

In a further embodiment, the chemical additive is a starch, a carboxymethyl cellulose, a polyacrylamide derivative, a dispersant, a debonding agent, cellulose fibrils, polyamide-epichlorohydrin, melamine, urea formaldehyde, or a polyimine.

In accordance to another embodiment, the method described herein further comprises chemically and/or heat treating the pulp after mechanically treating the pulp.

In another embodiment, the pulp is a combination of different pulps.

In accordance with the present disclosure there is provided an extensible paper comprising: pulp fibers having a length-weighted average curl index C_(m) of between 0.1 and 0.45; and a polymer in an amount of from 2 to 40 wt % based on the weight of the extensible paper; wherein the extensible paper has a Gurley air resistance below 20 s/100 mL and an elongation at break of at least 7%.

In a particular embodiment, the elongation at break is from 10% to 90%.

In a particular embodiment, the extensible paper is a polymer-impregnated extensible paper.

In a particular embodiment, the curl index of the fibers contained in the extensible paper is of from 0.25 to 0.45.

In a particular embodiment, the impregnated polymer is present in an amount of 10 wt %.

In another embodiment, it is provided an extensible paper produced by the method as described herein.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a flowchart showing a method for producing an extensible paper according to an embodiment, wherein optional steps are identified with dashed lines.

FIG. 2A to 2D are micrographs showing the fiber morphology modifications induced by mechanical treatments of a never-dried-unbleached kraft pulp; wherein in (A) untreated fibers, (B) mechanically treated fibers with higher fiber curl, (C) Refined fibers with surface fibrillation and (D) fibers subjected to refining followed by mechanical curl inducement with both surface fibrillation and high curl.

FIG. 3 is a graph showing the curl index (C_(LW)) of mechanical (TMP), chemi-mechanical (BCTMP), at low and high freeness, and kraft pulps, made with various wood species (mainly black spruce, aspen or Douglas fir), as a function of the pressure applied (bars) during treatment in plug screw.

FIG. 4 is a graph showing the number of kinks (per mm) of mechanical (TMP), chemi-mechanical (BCTMP), at low and high freeness, and kraft pulps, made with various wood species (mainly black spruce, aspen or Douglas fir), as a function of the pressure applied (bars) during treatment in plug screw.

FIGS. 5A and 5B are graphs showing the Water Retention Value (WRV in g/g), wherein in (A) mechanically treated chemical pulp fibers and (B) mechanically treated mechanical pulp fibers, as a function of the pressure applied (bars) during treatment in plug screw, with and without heat treatment;

FIGS. 6A to 6D are bar charts showing the elastic modulus (A), tensile index (B), elongation at break (C) and specific volume (D) of standard handsheets made from an unrefined NBSK pulp and handsheets made from the same NBSK pulp after mechanical treatment in a plug screw.

FIGS. 7A to 7D are bar charts showing the elastic modulus (A), tensile index (B), elongation at break (C) and specific volume (D) of handsheets made from an unrefined NBSK pulp and impregnated with 10 wt % of an acrylic copolymer and handsheets made from the same NBSK pulp after mechanical treatment in a plug screw and impregnated with 10 wt % of an acrylic copolymer.

FIG. 8 is a graph showing the tensile index (N.m/g) as a function of the elongation at break (%) of extensible papers in accordance with an embodiment.

FIG. 9 is a schematic illustration showing a system for producing fibers suitable for making extensible papers in accordance with an embodiment.

FIG. 10 is a graph showing the forming depth (mm) as a function of the elongation at break (%) of an extensible paper in accordance with an embodiment.

DETAILED DESCRIPTION

It is provided the manufacture of highly extensible, lignocellulosic fiber-based papers (hereafter referred to as “extensible paper”). The flowchart in FIG. 1 illustrates possible embodiments for the method for manufacturing the extensible papers that is presented below and encompassed herein.

The extensible paper comprises pulp fibers having an initial, or first, length-weighted average curl index C_(LW1). The fibers are provided in pulp form, which can have a consistency (percentage of oven dry mass in the pulp) of above 20% relative to the mass of the pulp. In some embodiment, the pulp has a consistency of between 20% and 50%.

Curl index and kinks were measured using the Fiber Quality Analyzer (FQA) from OpTest Equipment Inc. Curl index (C): is defined as the ratio of the true contour length L of the fiber divided by the projected length I of the fiber minus

$1\mspace{14mu} {\left( {C = {\frac{L}{l} - 1}} \right).}$

The curl index is calculated for each individual fiber. A curl index of zero indicates that no curl is present. The term “length-weighted curl index” or “average length-weighted curl index” as used herein (hereafter referred to as “curl index”) represents the mean curl index length-weighted (C_(m)) reported by FQA as the sum of individual C of each fiber multiplied by its contour length divided by the summation of the

$\left( {C_{LW} = \frac{\Sigma \; {C_{i} \cdot L_{i}}}{\Sigma \; L_{i}}} \right).$

The first length-weighted average curl index (C_(LW1)) can be between 0 and 0.3. Therefore, the fibers can be free of curl and/or kink or can be slightly curly. Preferably, the initial length-weighted average curl index is of 0 to 0.15.

The pulp fibers can be mechanical, chemi-mechanical and chemical pulps. In a particular embodiment, the pulp fibers are chemical pulp fibers produced by the kraft process. The kraft fibers can contain significant amounts of residual lignin as indicated by a high Kappa number.

In a particular embodiment, and as shown in FIG. 1, at least a portion of the lignocellulosic pulp fibers 10 composing the extensible paper have been mechanically treated 13 to induce curl and/or kinks. Curly pulp fibers can be obtained by mechanical treatment 13, with no chemical addition 14, using high consistency pressurized refining installations (or high consistency pulp compression process) found in pulp and paper mills. In some embodiments, chemical additives 22 such as plasticizers can also be added to the pulp. In an embodiment, curl inducement is produced by subjecting the pulp fibers 10 to compressive and shear forces in a plug screw device. The plug screw device can be part of a pressurized refining system. Mechanical curl inducement can also be produced by using other devices such as hammer mill, Kollergang mill, BIVIS device, explosion pulping device or specialized custom-designed equipment such as revolving drum.

The mechanical treatment 13 induces fiber deformations such that the resulting pulp fibers have a second length-weighted average curl index (C_(LW2)) higher than the first average curl index. The second curl index can be between 0.1 and 0.45, preferably between 0.25 and 0.45. FIGS. 2A to 2D illustrate the changes in fiber morphology induced by mechanical treatment in the case of a never-dried unbleached kraft pulp. The changes in morphology result from compressive and shear forces exerted on the fibers during treatment.

The level of curl obtained, defined by the second curl index, will depend at least in part on the type of pulp, the extent of mechanical treatment and the wood species. According to FIGS. 3 and 4, which show the change in curl index and number of kinks per mm, respectively, with increasing treatment pressure for different mechanical, chemi-mechanical and chemical pulps, a higher curl index is achieved with kraft pulp as compared to mechanical pulps. The term “kink index” (KI) as used herein refers to the sum of the number, N_(x), of kinks (an abrupt change in fiber curvature) within a range of “x” kink angles [deg], divided by the total fiber length of all the fibers, L_(T). The calculation of kink index by FQA gives more weight on higher angles

$\left( {{KI} = \frac{{2N_{({21 - 45})}} + {3N_{({46 - 90})}} + {4N_{({91 - 180})}}}{L_{T}}} \right).$

The term “kinks per mm” is calculated by dividing the total number of kinks, N_(x), by total fiber length, L_(T).

FIG. 3 also shows that for kraft pulp fibers, the curl index reaches a plateau above a pressure of about 6 bars in the plug screw. It has been found that, while curl is typically a good indicator of stretch potential, the elongation at break can increase with increasing treatment pressure, even in the region where curl has reached a plateau. Without being bound to theory, it is suggested that, for certain pulp fibers, other mechanisms than curl also play a role in affecting the paper stretch. For example, in case of kraft fibers, structural wall micro-compressions can affect paper stretch, and such micro-compressions may increase as the kraft fibers are compressed and sheared in the plug screw.

The degree of fiber deformation (i.e. the curl index or kink index) resulting from the mechanical treatment can be controlled through the amount of heat and pressure supplied during the process. In a particular embodiment, the system pressure of the mechanical treatment is between 2 to 20 bars, preferably 3 to 8 bars and most preferably 3 to 6 bars. The specific energy consumed in the high consistency process, such as plug screw process, is about 60-100 kWh/tonne.

Still referring to FIG. 1, the pulp fibers 10 can be heat treated 14 after the mechanical treatment. Heat treatment 14 can lead to additional cross-linking (chemical bonds) within the cell wall of mechanically treated fibers. Structural features of fibers (such as curls and kinks) can be made more permanent and the water resistance of the fibers can be improved with an additional cross-linking of the individual fibers. Therefore, additional cross-linking can be desirable in applications requiring higher water resistance. In a particular embodiment, the heat treatment process comprises maintaining the mechanically treated pulp fibers (previously dried to less than 5% moisture content) in a chamber with recirculating air at elevated temperature for a certain amount of time. The time of the heat treatment can be of from 3 to 10 min. As shown in FIGS. 5A and 5B, wherein the evolution of water retention values (WRV) of chemical pulp fibers (FIG. 5A) and of mechanical pulp fibers (FIG. 5B) that were mechanically treated, with (●) and without (□) heat treatment are compared, it has been found that WRV decreases after heat treatment for both mechanical and chemical pulp fibers.

A chemical treatment 14 can also be used, in place of or additionally to the heat treatment (see FIG. 1), and to induce additional cross-linking within individual fibers. This typically leads to the closing of micro-structural pores within the cell wall of the fibers and to the lowering of the water retention value. Table 1 shows the WRV for mechanical and chemical fibers after combined mechanical and chemical treatments to induce curl. A chemical additive 22 can also be added in the pulp 10 prior to the mechanical treatment 13.

TABLE 1 Water retention value (WRV) of mechanical and chemical pulp fibers after combined mechanical and chemical treatments to induce curl WRV (g water/g Pulp condition dry pulp) Mechanical pulp (100% BCTMP, no treatment) 0.973 SBSK (southern pine kraft pulp, no treatment) 0.837 NBSK pulp (no treatment) 0.791 Mechanically curled NBSK pulp (14 bars) 0.620 Chemically curled & cross-linked NBSK 0.190 (harvested from a commercial diaper)

In an embodiment, after mechanical treatment 13 (for example by plug screw), the pulp fibers are transferred through a steaming tube. The residence time in the steaming tube is 2 to 4 minutes and preferably 2 minutes. After the steaming tube, pulp fibers are blown out of the refiner, while the refiner disks are kept fully open.

The pulp fibers 10 can be refined prior to the mechanical treatment 13. For example, referring to FIG. 1, the pulp fibers 10 can be fed to a refiner 12 at consistencies above 20% and, after refining, the pulp is fed to the plug screw 13. During the refining process, varying refining specific energies can be used to produce pulps with a range of strength properties.

Once the fibers are treated, by mechanical treatment 13 and optionally by pre-refining 12, chemical and/or heat treatment 14, a wet web is formed with the treated pulp fibers after blending 15 and using a papermaking process 16. The term “wet web” as used herein can be understood as a sheet of pulp formed by even distribution of a pulp on a surface.

The wet web is then dried 17 under restraint to form a dried web. As mentioned above, the amount of shrinkage incurred by the paper depends on the restraint applied during drying, so that wet web dried without restraint will shrink more than wet web dried fully restrained. The drying restraint can be applied to the wet web by applying in-plane tension, mechanically fixing its edges to avoid dimensional changes in the plane of the web during drying or by any other restraining or compressing means.

The method for manufacturing the extensible paper then comprises the addition to the dried web of a polymer 19. Synergistic interactions between the treated fibers and thermoplastic polymer in the paper can then occur and provide high stretch. The polymer is preferentially soft and stretchy. The polymer can be bio-based, biodegradable and/or petroleum-based. Examples of biodegradable polymers include polylactic acid, polydroxyalkanoates and natural latex (Hevea Brasiliensis). Examples of petroleum-based polymers include thermoplastics such as polyethylene, polypropylene and polyethylene terephthalate. The petroleum-based polymer can also be selected from synthetic latexes such as styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, acrylic, vinyl acetate polymers and copolymers, and vinyl chloride-vinylidene chloride copolymers.

After the papermaking and drying processes of the web prepared with the mechanically treated fibers, the polymer can be added as a water-based dispersion to the base paper by impregnation, coating, sizing or spraying. In a preferred embodiment, the polymer is a latex with low glass transition temperature added to the paper by impregnation. The term “impregnation” as used herein comprises flooding the web with a water-based polymer, removing the excess polymer from the surface of the web and drying the web in order to evaporate the water. An optional calendering step 18 may occur before the addition of the polymer 19 or after, in order to notably compact the paper surface to improve for example its smoothness and gloss.

The combination of mechanically treated fibers and addition of polymer provides a composite material having properties that are intermediate between those of a paper and those of a plastic. High stretch potential is imparted to the fibers during mechanical treatment and the low elastic modulus of the paper allows the load applied during tensile testing to be shared between the pulp fibers and polymer in the sheet.

In an embodiment, the polymer is added in an amount of from 2 to 40 wt % of the extensible paper. Preferably, 5-20 wt % of polymer is added.

FIGS. 6A to 6D compare the elastic modulus (FIG. 6A), tensile strength (FIG. 6B), elongation at break (FIG. 6C) and specific volume (FIG. 6D) of standard handsheets made from a Northern Bleached Softwood Kraft (NBSK) and from the same NBSK pulp after mechanical treatment in a plug screw under an applied pressure of 8 bars. While mechanical treatment of the pulp fibers increases their stretch potential, FIGS. 6A to 6C show that handsheets made of such mechanically treated fibers undergo large decreases in elastic modulus (stiffness) and tensile strength and a small decrease in elongation at break. The large decrease in tensile strength might be due to a reduction in the total fiber to fiber bonded area and to the presence of kinks in the fibers that cannot transmit forces across the network during tensile loading. It will be understood that such decrease in tensile strength prevents any gain in stretch potential to be realized in a paper made from the mechanically treated pulp fibers without the addition of the polymer.

According to FIG. 6D, mechanical treatment of the pulp fibers leads to a significant increase in specific volume or porosity. The extensible paper can therefore be used in applications requiring both high extensibility and high permeability.

As illustrated by FIGS. 7A to 7D, an increase in elongation at break can be observed, even at low dosages of the polymer, and depends on the type polymer added and the method used. In the examples of FIGS. 7A to 7D, 10% by weight of an acrylic copolymer (Acronal LA 471 S from BASF) was added by impregnation to basesheets made from the original NBSK pulp or from the NBSK pulp after mechanical treatment. The total grammage, including pulp fibers and polymer, was 60 g/m² in both cases. While the addition of the polymer can increase the stretch of basesheets made from the original NBSK pulp from 2.3% to 5.1%, the addition of the polymer to basesheets made from the mechanically treated NBSK pulp can increase the stretch from 2.0% to 14.8%.

As shown in FIG. 1, a polymer 21 can also be added to the pulp prior to papermaking. In that case, the polymer can be in the form of fiber or as a water-based dispersion (latex). If the polymer is added to the pulp as a latex, then a retention aid can also be added to the pulp to help retain the polymer during the papermaking process.

In a particular embodiment, the method includes the addition of strength additives within the pulp. The strength additives improve the strength properties of the extensible paper. The strength additive can include dry-strength agents, such as starch, polyacrylamide derivatives and cellulose fibrils. Examples of strength additive also include wet-strength agents such as polyamide-epichlorohydrin, melamine, urea formaldehyde and polyimines. Other additives, such as sizing agents, debonders, dispersants, cross linking agent, or cellulose fibrils, can also be added to the pulp prior to papermaking process.

Strength additives can be added to the pulp alone or together with retention aids that help retain the strength additive in the wet web during papermaking. As mentioned in the examples below, basesheets made from the mechanically treated NBSK pulp prepared in presence of 1% by weight of polyamide-epichlorohydrin resin, and impregnated with 10% by weight of an acrylic copolymer (Acronal LA 471 S), presented a dry and wet tensile strengths of 11.6 and 4.87 N.m/g, respectively. By comparison, the dry and wet tensile strengths of basesheets prepared without the strength additive but impregnated with latex were 9.7 and 1.05 N.m/g, respectively. Therefore, the presence of a strength additive in the extensible paper increases its dry and wet tensile strengths. The elastic modulus and the burst strength of the extensible paper are also increased by the addition of a strength additive.

As shown in FIG. 1, more than one pulp can be included in the composition of the extensible paper. The optional additional pulp 24 can include regular chemical, mechanical and high-yield pulps and other natural fibers as well as man-made fibers dispersible in water. The fibers in the additional pulp can also be mechanically treated 13.

In a particular embodiment, the extensible paper is manufactured in a continuous papermaking process. However, the polymer addition can also be performed offline during converting operations.

The extendible paper as described herein comprises at least mechanically treated pulp fibers obtained by the above-defined treatment, and a polymer defined herein in an amount of from 2 to 40 wt % based on the weight of the extendible paper. The polymer is preferably added by impregnation, as described herein, so that the extensible polymer is preferably a polymer-impregnated extensible paper, having a polymer content of between 2 to 40 wt %, preferably 10 wt %. The length-weighted average curl index C_(LW) of the mechanically treated pulp fiber is of at least 0.1. The curl index can be of 0.1-0.45, preferably 0.2-0.45, and more preferably 0.25-045. The extensible paper can also comprise other components such as strength additives, a cross-linking agent, retention aids, or other type of pulps added during the manufacturing process.

In a particular embodiment, the extensible paper is dried under restraint and a polymer has been added after drying. The combination of mechanical treatment, restrained drying and addition of polymer after drying leads to a Gurley air resistance below 20 s/100 mL while providing high extensibility. The porosity and elongation at break of the resulting extensible paper are increased and shrinkage is decreased. The elongation at break can be of at least 10%, more particularly of 10 to 90%. The specific properties of the extensible paper, obtained by the combination of the method steps identified above, allows extending the use of paper to non-traditional applications, and provides a green alternative to plastic products, while using existing pulp and paper manufacturing processes.

The properties of the paper can be tailored to meet a wide range of end-use requirements. For example, the paper can have a relatively high elongation at break and relatively low strength, and therefore have properties similar to that of plastic materials. The paper can also have high tensile strength and an elongation at break of at least 10%.

The extensible paper can be used in applications that include: mulch film for horticultural applications, sack paper and bag for packaging applications, and barrier or wrapping material for protecting items such as wood lumber, paper rolls, home furniture, building materials and thermoformed packaging products. The extensible paper can also be a high grammage paperboard. For example the extensible paper can have a grammage of from 25 to 300 g/m². In this case, the high grammage paperboard can be moulded into 3D shapes suitable for various packaging applications.

The properties of the extensible paper (wet and dry tensile strengths, tear strength, burst strength, tensile energy absorption and Gurley air resistance) can be controlled by changing the manufacturing process and/or the composition of the extensible paper. For example, it is possible to (i) vary the proportion of mechanically treated fibers, (ii) vary the amount and type of polymer in the extensible paper, (iii) vary the type of pulp that is mechanically treated, (iv) vary the level of induced curl (the second curl index), (v) vary the refining energy and occurrence through mechanical treatment, (vi) add other pulps and (vii) use additives such as wet and dry strength agents, retention aids and sizing agents. More particularly, possible approaches to increase elongation at break include: increasing the amount of polymer in the composition of the paper, using a polymer with higher stretch potential, increasing the pressure applied to the pulp during mechanical treatment. Possible approaches to increase their tensile and burst strengths include refining the pulp before or after mechanical treatment, adding one or several stronger pulps, adding strength additives to the paper, using a polymer with higher strength properties.

FIG. 8 illustrates the wide range of tensile strength and elongation at break achieved with various embodiments of the extensible paper.

The present disclosure will be more readily understood by referring to the following examples, which are given to illustrate the disclosure rather than to limit its scope.

Description of Methods

Preparation of mechanically curled fibers

Different types of pulps including chemical (kraft), mechanical (TMP) and chemi-thermo-mechanical (CTMP) or bleached chemi-thermo-mechanical (BCTMP) were used to produce curly fibers for different applications.

A pilot plant pressurized refining system was used, as illustrated in FIG. 9, to produce fibers with structural curl and kinks. The system comprised a chip/fiber bin 30, plug screw 32, steaming tube feeding system 34, a 22″ single disk refiner 36 and cyclone 40. The pressure of the system was varied from 3 to 16 bars in order to produce fibers with different curl and kink index values. Steaming time of 2 to 4 minutes was covered and the pulp consistency was varied from 28% to 50%. The wood species included Canadian eastern black spruce, balsam fir, Canadian western spruce-lodgepole pine and balsam fir (SPF), aspen, and Douglas fir. The fiber curl and kink indexes were measured using a Fiber Quality Analyzer (FQA).

Determination of Water Retention Value (WRV)

The following procedure was used to measure the water retention value (WRV) of pulps. The pulp was dispersed in water at about 1% consistency. Then the dispersed pulp was drained over a 150 mesh inside a deckle. The resulting pulp pad was separated in three subdivided-pads and a portion of each subdivided-pad was added in one of three specially-fitted centrifuge tubes. The loaded centrifuge tubes were taken in pairs and adjusted to the same weight by adding a little water to the lighter one. Each pair was mounted in opposite holes in the centrifuge rotor and the uncapped tubes were centrifuged at 900 g (g=Earth's gravitational force) for 30 minutes. The pulp pad portions were then transferred from the tubes to pre-weighted bottles. During the transfer, the pulp pad portions were broken into half a dozen small pieces. Stoppers were installed on the bottles and each bottle with the stopper was reweighed. The bottles and their content were then put to dry at 105° C. After drying for at least 4 hours in the oven, the weight of the bottles was measured and the WRV calculated based on the three subdivided-pads.

Impregnation of basesheet with latex:

Synthetic latex was added to the basesheet using the following procedure. (1) The basesheet is first placed between two mesh holders with square holes 4 mm a side. (2) The two mesh holders and basesheet were completely immersed in a bath of latex for 1 minute. (3) The mesh holders and basesheet were retrieved from the bath and placed over a blotter lying flat on a counter. (4) The top mesh was removed and the basesheet was covered with a piece of satin. A second blotter was placed on top of the satin. (5) The whole assembly was flipped over so that the blotter originally in contact with the counter was on top. The blotter and mesh were removed and the side of the basesheet that was then exposed was covered with another piece of satin and then another blotter. (6) A couching roll weighing 28 lbs. was passed twice (back and forth motion) over the assembly. (7) The top blotter and piece of satin were removed and the basesheet was peeled from the bottom piece of satin. (8) The basesheet was placed against a drying plate covered with Teflon and the drying plate and handsheets were placed into the recessed rim of a drying ring, such that the handsheet was in contact with the surface of the recessed rim. Several drying rings were stacked in such a way that each handsheet was facing up. (9) The stack of drying rims was loaded with a 10 lbs weight to insure uniform drying restraint over the area of the handsheet(s) and to prevent shrinkage. (10) The amount of latex thus added to the basesheet can be controlled by adjusting the solids content of the latex bath. In general, the amount of polymer transferred to the basesheet increases nonlinearly with the solids content of the bath. For each type of latex tested, that relationship was obtained experimentally by first repeating the procedure detailed above three or four times, varying the solids content of the bath each time. A quadratic fit to the data was then used to determine the solids content to use in subsequent experiments in order to achieve a given addition level.

Analysis of handsheets

Analysis of handsheets prepared according to the present method, with or without polymer addition, was performed according to the PAPTAC standard methods of Table 2 below.

TABLE 2 PAPTAC standard methods for analysing handsheets Method Reference Grammage of paper and paperboard PAPTAC Standard D.3 Thickness and apparent density of paper PAPTAC Standard D.4 and paperboard Tensile breaking properties of paper and PAPTAC Standard D.34 paperboard (Constant rate of elongation method) Moisture determination for chemical PAPTAC Standard G.3 and physical analysis Bursting strength of paper PAPTAC Standard D.8 Air resistance of paper PAPTAC Standard D.14 Internal tearing resistance of paper, PAPTAC Standard D.9 paperboard and pulp handsheets

To measure the tensile properties of rewetted handsheets, the handsheets were first immersed in water for 1 minute and then placed between two blotters to remove the excess water. Tensile breaking properties of the rewetted handsheets were then measured according to Standard D.34.

Measure of formability

An in-house method was developed to form paper into three-dimensional shapes. The device involves a spherical die attached to an Instron testing system. The spherical die was lowered at a specified speed into a paper sample clamped between two plates so that no sliding of the paper into the forming cavity could take place. The total displacement of the sphere at failure provided a measure of formability. Forming ratios were defined as the forming depth over the base diameter.

EXAMPLE I

This example illustrates how the properties of a paper a described herein, made from a Northern Bleached Softwood Kraft (NBSK) pulp consisting of black spruce fibers, can be controlled by mechanical treatment, by impregnation of latex and by addition of polyamide-epichlorohydrin (PAE). In total, five different sets of handsheets were prepared and their composition is given in Table 3. In addition, the mechanical properties measured on the first and second sets of handsheets are shown on the first two lines of Table 4.

TABLE 3 Composition of five sets of handsheets Set # Pulp Additives 1 Northern Bleached Softwood Kraft — (NBSK) 2 NBSK 10% Acronal LA 471S 3 NBSK after mechanical treatment (8 bars) — 4 NBSK after mechanical treatment (8 bars) 10% Acronal LA 471S 5 NBSK after mechanical treatment (8 bars) 10% Acronal LA 471S 1% polyamide- epichlorohydrin

TABLE 4 Mechanical properties of the five sets of handsheets Tear Burst Wet Tensile TEA Elastic Index Index Tensile Set Caliper Stretch Index Index Modulus (mN (kPa Index # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) (N · m/g) 1 118 2.3 31.2 520 485 25.8 1.90 1.47 2 119 5.1 31.7 1230 320 21.0 4.17 2.72 3 145 2.0 10.6 150 185 10.5 0.60 0.51 4 155 14.8 9.7 1200 85 24.0 2.19 1.05 5 145 23.4 11.6 2300 105 23.4 2.78 4.87 the target grammage was 60 g/m² in all cases and TEA stands for Tensile Energy Absorption

The first set (#1) corresponds to standard handsheets (of grammage 60 g/m²) made from the original pulp.

The second set (#2) of handsheets of grammage 54 g/m² was prepared from the same pulp. The handsheets of the second set were dried under restraint and then impregnated with an ester acrylate copolymer (Acronal LA 471S, from BASF) using the procedure described herein. The target grammage of the paper, including pulp fibers and latex, was 60 g/m². This corresponds to a latex content of 10% by weight. As shown by the results of Table 4 (lines 1 and 2), the addition of latex led to an increase in the elongation at break of from 2.3 to 5.1%. It also had a positive impact on burst and Tensile Energy Absorption (TEA) indices as well as on wet tensile strength. The elastic modulus decreased from 485 to 320 km.

The third set (#3) of standard handsheets was prepared from the same commercial NBSK pulp that was treated mechanically in a plug-screw feeder to induce deformations and curl in the fibers. The pressure applied to the pulp during the mechanical treatment was 8 bars. The sheets prepared from the treated fibers were bulky and weak. As shown by the results of Table 4 (line 3), mechanical treatment of the pulp led to a large decrease in all strength properties of the paper. In particular, the tensile strength decreased from 31.2 to 10.6 N.m/g and the elongation at break from 2.3% to 2.0%. The elastic modulus also decreased from 485 km to 185 km while caliper increased from 118 to 145 μm.

The fourth set (#4) of handsheets was prepared from the mechanically treated pulp, this time at a grammage of 54 g/m². These handsheets were then impregnated with 10 wt % of the latex Acronal LA 471S. As shown by the results of Table 4 (line 4), the impregnated handsheets had an elongation at break of 14.8%, which represents more than seven time the elongation at break of the same handsheets without latex. The TEA index, which is a measure of how much energy must be expended to break the paper, also increased from 150 to 1200 mJ/g.

The fifth set (#5) of handsheets was produced from the mechanically treated fibers of the third set, but here 1 wt % of polyamide-epichlorohydrin (PAE), a commonly used wet-strength agent, was added to the pulp prior to making handsheets of grammage 54 g/m². The handsheets of the fifth set were then impregnated with 10 wt % of the latex Acronal LA 471S. As shown in line 5 of Table 4, addition of PAE to the pulp had a large positive impact on the dry and wet tensile strengths as well as on the tensile energy absorption of the sheets impregnated with latex.

EXAMPLE II

This example illustrates how the properties of a paper as described herein, made from a never-dried unbleached softwood kraft (UBSK), can be controlled by mechanical treatment, by impregnation of latex and by addition of polyamide-epichlorohydrin (PAE). In total, four different sets of handsheets were prepared for this example.

The first set (#1) corresponds to standard handsheets prepared from the original UBSK pulp (#1). The second set (#2) corresponds to handsheets made from the UBSK pulps but with 1% PAE added to the pulp prior to sheetmaking and 10% latex added to the dry sheets by impregnation. The third set (#3) corresponds to standard handsheets prepared from the UBSK pulp after treatment in a plug-screw feeder at a pressure of 3.5 bars (#3). The fourth set (#4) corresponds to handsheets made from the UBSK pulps after treatment in a plug-screw feeder at a pressure of 3.5 bars and with 1% PAE added to the pulp prior to sheet making and 10% latex added to the dry sheets by impregnation.

The composition and mechanical properties of the different sets of handsheets are summarized respectively in Tables 5 and 6 below. A comparison of data from the first and fourth sets (#1 and #4) shows that mechanical treatment of the pulp combined with the addition of PAE and latex leads to large increases in stretch, tensile energy absorption and wet tensile strength. The handsheets of the fourth set (#4) also have a much lower modulus and they are weaker in the dry state than sheets made from the original pulp (tensile index of 30.2 N.m/g compared to 48.3 N.m/g).

TABLE 5 Composition of the four sets of handsheets Set # Pulp Additives 1 Unbleached Northern Softwood — Kraft (UBSK) 2 UBSK 10% Acronal LA 471S 1% polyamide-epichlorohydrin 3 UBSK after mechanical — treatment (3.5 bars) 4 UBSK after mechanical 10% Acronal LA 471S treatment (3.5 bars) 1% polyamide-epichlorohydrin

TABLE 6 Mechanical properties of the four sets of handsheets Tear Burst Wet Tensile TEA Elastic Index Index Tensile Set Caliper Stretch Index Index Modulus (mN (kPa Index # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) (N · m/g) 1 117 2.3 48.3 760 620 27.3 3.11 1.90 2 115 4.2 67.8 1800 410 10.8 7.07 18.0 3 141 1.8 14.8 200 230 16.7 0.90 0.53 4 130 9.7 30.2 2300 230 26.7 4.00 8.86 The target grammage was 60 g/m² in all cases

EXAMPLE III

This example illustrates how the properties of the paper encompassed herein can be controlled by varying the amount of polymer in the sheet. The starting pulp in this case was a Northern Bleached Softwood Kraft pulp (NBSK) different from the one used in Example II. In total, five different sets of handsheets were prepared for this example.

The first set (#1) corresponds to standard handsheets made from the original NBSK pulp, the second set (#2) corresponds to standard handsheets made from the pulp after mechanical treatment in a plug-screw feeder at 12 bars, the third, fourth and fifth set corresponds to handsheets made from the mechanically treated pulp and impregnated with 10% (#3), 20% (#4) or 30% (#5) of latex (Acronal LA 471S). The grammage of the handsheets (including pulp fibers and latex) was 60 g/m² in all cases. The composition and mechanical properties of the different sets of handsheets are summarized in Tables 7 and 8, respectively.

As reported in Examples II and III, plug-screw treatment alone had a negative impact on all mechanical properties, including elongation at break, which decreases from 2.5 to 1.7%. However, highly extensible papers were produced when the handsheets made from the mechanically treated fibers were subsequently impregnated with latex. The elongations at break of measured on handsheets containing 10%, 20% and 30% latex, were of 27.2%, 46.2% and 61.8% respectively.

TABLE 7 Composition of the five sets of handsheets Set # Pulp Additives 1 Northern Bleached Softwood Kraft (NBSK) — 2 NBSK after mechanical treatment (12 bars) — 3 NBSK after mechanical treatment (12 bars) 10% Acronal LA 471S 4 NBSK after mechanical treatment (12 bars) 20% Acronal LA 471S 5 NBSK after mechanical treatment (12 bars) 30% Acronal LA 471S

TABLE 8 Mechanical properties of the five sets of handsheets Burst Tensile TEA Elastic Tear Index Set Caliper Stretch Index Index Modulus Index (kPa # (μm) (%) (N · m/g) (mJ/g) (km) (mN m²/g) m²/g) 1 123 2.5 23.1 400 390 19.2 1.51 2 155 1.7 7.5 95 130 7.6 0.35 3 191 27.2 7.4 1700 41 17.5 1.32 4 165 46.2 8.9 3450 30 18.8 2.41 5 158 61.8 9.3 4600 22 15.6 2.59 The target grammage was 60 g/m² in all cases

EXAMPLE IV

This example illustrates the impact of changing the pressure applied to the pulp during the mechanical treatment on the final properties of the stretchable paper. The starting pulp was the same NBSK pulp used in Example IV. In total, three different sets of handsheets were prepared, all of them impregnated with 10% of a styrene-butadiene latex (CP 638NA, manufactured by Dow Chemical).

The handsheets of the first set (#1) were prepared from the original NBSK pulp while those of the second (#2) and third (#3) sets were made from the original NBSK pulp after treatment in a plug-screw feeder at pressures of 6 bars (#2) and 16 bars (#3). The composition and mechanical properties of the different handsheets are given in Tables 9 and 10 below.

The results from Table 10 show that the elongation at break of the handsheets impregnated with latex increases with the amount of pressure applied to the pulp during the mechanical treatment. By contrast, the mechanical properties such as tensile strength and burst resistance decrease when the applied pressure increases.

TABLE 9 Composition of the three sets of handsheets Set # Pulp Additives 1 Northern Bleached Softwood Kraft (NBSK) 10% CP 638NA 2 NBSK after mechanical treatment (6 bars) 10% CP 638NA 3 NBSK after mechanical treatment (16 bars) 10% CP 638NA

TABLE 10 Mechanical properties of the three sets of handsheets Burst Tensile TEA Elastic Tear Index Set Caliper Stretch Index Index Modulus Index (kPa # (μm) (%) (N · m/g) (mJ/g) (km) (mN m²/g) m²/g) 1 132 5.6 31.0 1200 260 18.1 3.37 2 159 7.1 18.3 990 150 17.8 2.21 3 187 9.7 14.4 1080 90 12.9 1.78 The target grammage was 60 g/m² in all cases

EXAMPLE V

This example illustrates the impact of pulp refining prior to the mechanical treatment. The starting pulp in this case was a never-dried Unbleached Softwood Kraft pulp (UBSK) similar to that used in Example III. In the present example, the pulp was refined at high-consistency in a double-disk refiner prior to mechanical treatment in a plug-screw feeder. The amount of energy expended during refining was 1000 kWh/tonne while the subsequent mechanical treatment was performed at a pressure of 3.5 bars.

Tables 11 and 12 detail the composition and mechanical properties of the four sets of handsheets. The first set (#1) corresponds to standard handsheets prepared from the original UBSK pulp (#1). The second set (#2) corresponds to handsheets made from the UBSK pulps after refining treatment. The third set (#3) corresponds to handsheets prepared from the UBSK pulp after refining treatment and treatment in a plug-screw feeder at a pressure of 3.5 bars (#3). The fourth set (#4) corresponds to handsheets prepared from the UBSK pulp after refining treatment and treatment in a plug-screw feeder at a pressure of 3.5 bars with 10% latex added to the dry sheets by impregnation.

As shown in Table 12, refining of the pulp has a large impact on its strength so that even after plug-screw treatment, paper made from the treated pulp has a tensile index above 30 N.m/g. Handsheets made from the treated pulp and impregnated with 10% latex has an elongation at break of 11.9%, a TEA index 2,600 mJ/g and a Gurley air resistance of only 2.6 s/100mL. The skilled person will appreciate that the combination of high elongation at break, high toughness and high permeability makes the extensible paper suitable for sack grade applications.

TABLE 11 Composition of the four sets of handsheets Set # Pulp Additives 1 Unbleached Northern Softwood Kraft (UBSK) — 2 UBSK after refining — 3 UBSK after refining and mechanical treatment — 4 UBSK after refining and mechanical treatment 10% Acronal LA 471S

TABLE 12 Mechanical properties of the four sets of handsheets Tear Burst Tensile TEA Elastic Index Index Gurley Air Set Caliper Stretch Index Index Modulus (mN (kPa Resistance # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) (s/100 mL) 1 125 1.9 40.9 560 580 26.4 3.26 2.1 2 110 5.1 83.9 2900 680 17.3 7.24 430 3 135 4.2 33.6 1100 420 21.6 2.54 5.9 4 — 11.9 27.5 2600 240 22.3 — 2.6 The target grammage was 60 g/m² in all cases

EXAMPLE VI

It is illustrated how the properties of the stretchable paper as described herein change with the type of polymer added to the basesheet.

In total, ten different sets of handsheets were prepared in Example VII. The starting pulp for the first five set was the same NBSK pulp used in Examples IV and V. The first set (#1) was prepared from the NBSK pulp mechanically treated in a plug-screw feeder at a pressure of 16 bars. Basesheets made from the mechanically treated pulp but of lower grammage were also prepared for the second to fifth sets (#2 to #5). The basesheets were impregnated with four different latexes added in an amount of 10% by weight: CP 638NA (#2) , Styronal NX 4222X , BASF (#3), Acronal LA 471S (#4) and Acronal S504NA, BASF (#5). The composition and mechanical properties of the handsheets from the five sets are given in Tables 13 and 14, respectively.

The results from Table 14 show that the mechanical properties of the stretchable paper depend on the nature of the latex that is used. In this particular example, the latex used for the fourth set (#4) is the one that provides the highest elongation at break the latex used for the second set (#2) provides the highest increase in strength (relative to the base paper made from the mechanically treated fibers).

The starting pulp for sets #6 to #10 was also the same NBSK pulp than in Example II. In this case, the pulp was refined in a double disk refiner and then treated mechanically in a plug-screw feeder. The energy expended during refining was 1000 kWh/tonne while the pressure inside the plug-screw feeder during treatment was 3.5 bars. The basesheets of the seventh and eighth sets were impregnated with 10% (#7) and 20% (#8) of the synthetic latex Acronal LA 471S while those of the fourth and fifth sets were impregnated with 10% (#9) and 20% (#10) of a natural latex provided by the company Chemionics. Tables 13 and 14 show the composition and mechanical properties for the five sets of handsheets prepared herein.

The results of Table 14 show that while the elongation at break obtained with the natural latex is not as high as the elongation at break obtained with the synthetic one, it is still possible to produce an extensible paper that is essentially 100% bio-based and biodegradable and has an elongation at break (or stretch) of 10% or more at relatively low dosages of the natural polymer.

TABLE 13 Composition of the different sets of handsheets Set # Pulp Additives 1 NBSK after mechanical — treatment (16 bars) 2 same 10% CP 638NA 3 same 10% Styronal NX 4222X (BASF) 4 same 10% Acronal LA 471S 5 same 10% Acronal S504NA (BASF) 6 NBSK after refining and — mechanical treatment (3.5 bars) 7 same 10% Acronal LA 471S 8 same 20% Acronal LA 471S 9 same 10% natural latex 10 same 20% natural latex

TABLE 14 Mechanical properties of the different sets of handsheets produced. Tear Burst Tensile TEA Elastic Index Index Set Caliper Stretch Index Index Modulus (mN (kPa # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) 1 143 1.4 6.7 65 135 6.6 0.42 2 187 9.7 14.4 1100 680 12.9 1.78 3 170 11.5 6.9 650 60 15.5 0.71 4 181 17.5 9.3 1300 45 14.9 1.17 5 167 10.6 11.9 940 70 14.2 1.79 1 149 4.2 21.1 700 280 16.6 1.56 2 148 16.1 19.6 2600 175 18.8 2.47 3 151 34.0 16.7 4500 120 13.6 2.92 4 154 9.3 16.1 1200 130 18.1 1.78 5 145 12.0 17.2 1600 95 23.6 2.15 The target grammage was 60 g/m² in all cases

EXAMPLE VII

This example illustrates how extensible papers can be prepared from mixtures of two different pulps. It also illustrates the impact of latex content and type on the mechanical properties of the extensible papers. It finally shows how papers prepared in accordance with an embodiment can be moulded into three-dimensional shapes.

Two different pulps produced from the same commercial NBSK pulp were used in the example. The first pulp (P1) was produced by treating mechanically the NBSK pulp in a plug-screw feeder at a pressure of 3.5 bars. The second pulp (P2) was produced by refining the NBSK pulp in a double-disk refiner. The amount of energy expended during refining was 1000 kWh/tonne. Twelve different sets of handsheets were prepared from various mixtures of the two pulps. Nine of these sets included in their composition up to 20% by weight of latex added by impregnation to the basesheets. The composition and mechanical properties of the different sets of handsheets are shown in Tables 15 and 16, respectively.

TABLE 15 Composition of the different sets of handsheets Set # Pulp Additives 1 100% P2 — 2 50% P1, 50% P2 — 3 100% P1 — 4 100% P2 20% Acronal S 728 5 50% P1, 50% P2 10% Acronal S 728 6 50% P1, 50% P2 20% Acronal S 728 7 100% P1 20% Acronal S 728 8 100% P1 20% Acronal LA 471 9 50% P1, 50% P2 10% Acronal LA 471 10 50% P1, 50% P2 20% Acronal LA 471 11 100% P2 20% Acronal LA 471 12 100% P1 20% SNP S-1420-L Polyurethane dispersion

TABLE 16 Mechanical properties and formability of the sets of handsheets produced Tensile TEA Maximum Maximum Set Index Index Forming Depth load # Stretch (%) (N · m/g) (mJ/g) (mm) (N) 1 5.53 54.3 2223 11.3 185.86 2 4.7 27.7 1032 9.7 87.44 3 2.2 13.2 220 7.3 28.4 4 7.6 66.1 3254 13.8 295.8 5 7.6 40.0 2288 13.6 204.6 6 10.0 45.8 3081 14.8 252.0 7 11.8 31.6 2633 17.1 216.8 8 41.1 10.2 3335 25.5 98.2 9 13.9 22.3 2560 15.4 120.3 10 26.8 22.3 4796 19.6 170.2 11 15.3 38.6 4585 16.1 230.4 12 21.5 30.0 4802 20.8 319.9 Total grammage (including latex) was 80 g/m² in all cases

The results from Table 16 illustrate the wide range of mechanical properties that can be obtained by varying the composition of the paper. The elongations at break that were measured varied between 2.2% and 41.1%, with the highest elongations at break obtained with the mechanically treated pulp (P1) impregnated with 20% by weight of latex Acronal LA 471S (#8). By contrast, the highest tensile index was measured on the sample produced with pulp P2 and containing 20% by weight of Acronal S 728 (#4). According to Table 15, at the same latex addition level, the properties of the handsheets containing equal proportions of the two pulps were usually intermediate between those of the handsheets produced only with each of the pulps.

All the handsheets produced in Example VIII were also tested for formability by using a device developed in-house for that particular purpose. As described above, the forming depth data shown in Table 16 were obtained by first placing a spherical die (of diameter 9 cm) in contact with a paper sample clamped between two plates so that no sliding of the paper into the forming cavity could take place. The spherical die was then lowered at constant speed into the paper sample until the latter failed. The total displacement of the sphere during the experiment thus provided a measure of maximum forming depth for each sample. The maximum load applied to the sample during testing was also recorded in each case.

According to Table 16, the sample made form pulp P1 and containing 20% Acronal LA 471 (#8) had the highest formability, with a maximum forming depth of 25.5 mm. This corresponds to a forming ratio (defined as the ratio of forming depth to base diameter) of about 0.3. This is remarkably high for a paper formed in a process where no sliding is allowed (see FIG. 10). It illustrates the strong correlation between the maximum forming depth measured herein and the elongation at break measured in standard tensile testing.

EXAMPLE VIII

This example illustrates the impact of mechanical treatment and latex impregnation on handsheets prepared with thermo-mechanical pulp (TMP).

The thermomechanical pulp (TMP) was produced in the pilot plant in 3-stage refining using eastern black spruce wood chips. In the first stage, wood chips were refined in the pressurized 22″ single disk refiner (see FIG. 9). The consecutive stages were done in a 36″ double disc atmospheric refiner. The total refining specific energy was about 2482 kWh/t. In total, four sets of handsheets were produced from the TMP pulp. The composition for each set is given in Table 17.

The first and second sets correspond to standard handsheets (of grammage 60 g/m²) made from the original TMP pulps (#1) and from the TMP pulp after mechanical treatment (#2). For the third and fourth sets, handsheets of grammage 54 g/m² were first prepared from the original TMP pulps (#3) and from the TMP pulp after mechanical treatment (#4), dried under restraint and then impregnated with an ester acrylate copolymer (Acronal LA 471S). The target grammage of the paper, including pulp fibers and latex, was 60 g/m². This corresponds to a latex content of 10% by weight.

The mechanical properties of the handsheets were measured under traction in both dry and wet states and are presented in Table 18 below. Caliper, out-of-plane tear strength and burst resistance were also measured. According to the results of Table 18, the mechanical treatment of the TMP pulp decreased tensile strength while increasing stretch slightly. Table 18 also shows that latex addition to the basesheet increased the elongation at break from 2.14% to 3.73% in the case of the untreated TMP pulp, and from 2.37% to 4.83% in the case of the TMP pulp mechanically treated in the plug screw at a pressure of 6 bars.

TABLE 17 Composition of the four sets of handsheets Set # Pulp Additives 1 Thermo-mechanical pulp (TMP) — 2 TMP after mechanical treatment (6 bars) — 3 TMP 10% Acronal LA 471S 4 TMP after mechanical treatment (6 bars) 10% Acronal LA 471S

TABLE 18 Properties measured on the four sets of handsheets Tear Burst Wet Tensile TEA Elastic Index Index Tensile Set Caliper Stretch Index Index Modulus (mN (kPa Index # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) (N · m/g) 1 168 2.14 41.4 520 385 7.85 2.63 4.32 2 185 2.37 26.4 430 260 7.64 1.69 2.60 3 162 3.73 44.3 1090 306 6.21 3.75 4.46 4 173 4.83 29.4 1020 210 6.50 2.51 2.95 The target grammage was 60 g/m² in all cases. TEA stands for Tensile Energy Absorption

EXAMPLE IX

This example illustrates the impact of the amount of latex on handsheets prepared with thermo-mechanical pulp (TMP) and latex impregnation.

The thermomechanical pulp (TMP) was produced in the pilot plant in 3-stage refining using eastern black spruce wood chips. In the first stage, wood chips were refined in the pressurized 22″ single disk refiner to about 255 kWh/t (see FIG. 9). In the second and third stages, a 36″ double disc atmospheric refiner was used to refine the pulp to 1597 kWh/t and 1883 kWh/t, respectively. To induce further curl and kinks, the produced pulp was then treated in the pressurized refining system plug screw to a pressure of 16 bars. In total, three different sets of handsheets were prepared from that pulp and their composition is shown in Table 19.

The first set (#1) corresponds to standard handsheets of grammage 60 g/m² prepared form the TMP mechanically treated pulp. The second and third sets correspond to handsheets of grammage 54 g/m² and 42 g/m² prepared from the TTMP mechanically treated pulp impregnated with 10% (#2) and 30% (#3) of latex Acronal LA 471S using the procedure described herein. The target grammage of the paper, including pulp fibers and latex, was 60 g/m². The physical and mechanical properties measured on these three sets of handsheets are shown in Table 20.

According to Table 20 the elongation at break, TEA index and wet tensile index increased with increasing amount of latex, while the elastic modulus decreased.

TABLE 19 Composition of the three sets of handsheets Set # Pulp Additives 1 TMP after mechanical treatment (16 bars) — 2 TMP after mechanical treatment (16 bars) 10% Acronal LA 471S 3 TMP after mechanical treatment (16 bars) 30% Acronal LA 471S

TABLE 20 Properties measured on the three sets of handsheets Tear Burst Wet Tensile TEA Elastic Index Index Tensile Set Caliper Stretch Index Index Modulus (mN (kPa Index # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) (N · m/g) 1 199 2.05 4.61 68 59.8 2.13 N/A 0.72 2 206 7.06 9.27 480 52.6 2.94 0.76 1.69 3 166 18.5 7.24 1120 34.6 2.33 0.89 1.95 The target grammage (including fibers and polymer) was 60 g/m² in all cases

EXAMPLE X

This example illustrates how the point of addition of the latex impacts the mechanical properties of extensible papers as described herein.

A commercial NBSK pulp with a high content of Douglas fir fibers was used in the example. The pulp was first treated in a plug-screw feeder at 3.5 bars. Two different sets of handsheets were then prepared from the treated pulp. The first set (#1) of handsheets of grammage 54 g/m² was prepared from the treated pulp and dried under restraint. The dried handsheets were then impregnated with an ester acrylate copolymer (Acronal LA 471S, from BASF) using the procedure described herein. The target grammage of the paper, including pulp fibers and latex, was 60 g/m². This corresponds to a latex content of 10% by weight. The second set (#2) of handsheets of grammage 54 g/m² was also prepared from the treated pulp but handsheets from the second set were not allowed to dry prior to impregnation with polymer. Instead, handsheets from the second set were impregnated with the ester acrylate copolymer (Acronal LA 471S) after the pressing steps described in PAPTAC standard C.4. The target grammage of the paper, including pulp fibers and latex was also 60 g/m². The mechanical properties of papers from set #1 and set #2 are summarized in Table 21. Results from Table 21 show that both the elongation at break and tensile strength of papers from set #1 are higher than those of papers from set #2, indicating that, in order to achieve optimal performance, it is preferable to dry the handsheets prior to impregnation with latex.

TABLE 21 Properties measured on the two sets of handsheets Tear Burst Tensile TEA Elastic Index Index Set Caliper Stretch Index Index Modulus (mN (kPa # (μm) (%) (N · m/g) (mJ/g) (km) m²/g) m²/g) 1 156 13.4 11.4 1300 96 21.6 1.75 2 195 11.9 9.8 975 96 22.4 1.76 The target grammage (including fibers and polymer) was 60 g/m² in all cases

EXAMPLE XI

This example illustrates the impact on the final properties of the stretchable paper encompassed herein of changing: a) the pressure applied to the pulp during the mechanical treatment and b) the amount of polymer added to the sheet. The starting pulp for this example was a Northern Bleached Hardwood Kraft (NBHK) pulp comprising a majority of aspen fibers. In total, fourteen different sets of handsheets were prepared for this example. The grammage of handsheets prior to impregnation with latex was 60 g/m² in all cases.

The handsheets of sets #1 to #4 were prepared from the original NBHK pulp. Sets #5 to #8 were prepared from the NBHK pulp after treatment in a plug-screw at a pressure of 3.5 bars. Sets #9 to #11 were prepared from the NBHK pulp after treatment in a plug-screw at a pressure of 9 bars. Sets #12 to #14 were prepared from the NBHK pulp after treatment in a plug-screw at a pressure of 15 bars. The different sets also differ in the amount of latex added to the handsheets. The composition and mechanical properties of the different handsheets are given in Tables 22 and 23 below.

Results from Table 23 illustrate the large increase in elongation at break achieved by making handsheets from a mechanically treated pulp and subsequently impregnating the handsheet with an extensible polymer. Measured values of elongation at break increased with the amount of polymer added to the handsheet. For this pulp, applied pressure above 3.5 bars had no significant impact on stretch potential.

TABLE 22 Composition of the fourteen sets of handsheets Set # Pulp Additives 1 Northern Bleached — 2 Hardwood Kraft (NBHK)  5.4% Acronal LA 471S 3 10.1% Acronal LA 471S 4 20.3% Acronal LA 471S 5 NBHK after mechanical — 6 treatment (3.5 bars)  5.4% Acronal LA 471S 7 10.1% Acronal LA 471S 8 20.9% Acronal LA 471S 9 NBHK after mechanical —_ 10 treatment (9 bars) 10.4% Acronal LA 471S 11 19.4% Acronal LA 471S 12 NBHK after mechanical — 13 treatment (15 bars)  9.8% Acronal LA 471S 14 18.7% Acronal LA 471S

TABLE 23 Physical and mechanical properties of the different sets of handsheets Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 60.7 131 1.20 14.3 120 258 2 61.9 144 3.15 8.9 219 116 3 64.3 142 6.98 10.6 626 123 4 72.6 142 13.48 11.3 1291 84 5 61.1 149 1.27 10.5 96 182 6 59.9 151 5.66 6.8 326 77 7 63.6 154 11.00 7.8 737 71 8 73.9 154 28.18 8.4 1966 53 9 59.8 145 1.12 7.7 59 127 10 69.2 174 10.26 6.2 530 43 11 73.7 162 27.09 6.9 1584 40 12 59.2 157 0.96 5.5 35 97 13 68.7 184 12.76 5.6 595 36 14 73.1 170 26.88 5.7 1304 28

EXAMPLE XII

This example illustrates the impact of grammage on the final properties of the stretchable paper as described herein. The pulp used for this example was a Northern Bleached Softwood Kraft (NBSK) pulp treated mechanically in a plug-screw feeder at 3.5 bars. In total, eleven different sets of handsheets were prepared for this example.

The handsheets of sets #1 to #5 were prepared at a target grammage of 60 g/m² prior to impregnation with varying amounts of latex. Sets #6 to #8 were prepared at a target grammage of 40 g/m² prior to impregnation and sets #11 to #13 at a target grammage of 20 g/m², also prior to impregnation. The composition and mechanical properties of the different handsheets are given in Tables 24 and 25 below.

Results from Table 25 indicate that the increase in elongation at break with polymer content is similar for handsheets prepared at a target grammage of 40 or 60 g/m² (prior to impregnation). At the same polymer addition level, the elongation at break measured on the handsheets prepared at 20 g/m² was lower than that measured on handsheets prepared at the two higher grammages. Furthermore, at the same latex addition level, tensile index increased with handsheet grammage. However, the differences were less pronounced as polymer content was increased.

TABLE 24 Composition of the eleven sets of handsheets Set # Pulp Additives 1 NBSK treated mechanically at 3.5 bars — 2 (target grammage of 60 g/m² 5.40% Acronal LA 471S 3 prior to impregnation) 10.0% Acronal LA 471S 4 21.3% Acronal LA 471S 5 35.5% Acronal LA 471S 6 NBSK treated mechanically at 3.5 bars — 7 (target grammage of 40 g/m² 10.6% Acronal LA 471S 8 prior to impregnation) 16.7% Acronal LA 471S 9 NBSK treated mechanically at 3.5 bars — 10 (target grammage of 20 g/m² 11.5% Acronal LA 471S 11 prior to impregnation) 18.6% Acronal LA 471S

TABLE 25 Physical and mechanical properties of the different sets of handsheets of Example 13 Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 57.6 130 1.88 13.1 185 218 2 60.2 167 8.56 8.8 654 99 3 67.2 162 16.44 10.8 1522 90 4 77.9 165 37.35 12.4 3807 62 5 88.6 164 65.53 13.2 6850 46 6 39.1 104 1.28 9.1 82 159 7 41.7 118 18.38 9.9 1521 61 8 44.7 111 32.63 11.9 3180 59 9 20.8 69 1.43 4.6 42 57 10 24.8 85 13.57 8.4 951 41 11 24.7 78 26.78 9.7 2233 40

EXAMPLE XIII

This example illustrates that man-made fibers can be added to the furnish from which the stretchable paper as encompassed herein is made. The pulp used for this example was a Northern Bleached Softwood Kraft (NBSK) pulp treated mechanically in a plug-screw feeder at 3.5 bars. The man-made fibers were bicomponent fibers (bico) of length 6 mm and linear density 1.3 dTex comprising a PET core and PE sheath (T455, manufactured by Trevira). Four different sets of handsheets were prepared for this example.

The handsheets of set #1 were made from a mixture consisting of 80 wt % treated NBSK pulp and 20 wt % bicomponent fibers. The handsheets of set #2 were made from the same mixture as set #1 but they were subsequently subjected to a heat treatment to melt the outer layer of the bicomponent fibers. The handsheets of set #3 were made from the same mixture as set #1 but they were subsequently impregnated with latex. The handsheets of set #4 were also made from the same mixture as set #1 but they were subsequently subjected to the same heat treatment as set #2 and then impregnated with latex. All handsheets were prepared at a target grammage of 60 g/m² prior to impregnation, and no wet-pressing pressure was applied during the sheet making process. The composition and mechanical properties of the different handsheets are given in Tables 26 and 27 below.

Results from Table 27 indicate that heat treatment is required to develop the potential of bicomponent fibers. Results also illustrate how, in certain embodiments, a combination of bicomponent fibers and impregnation with latex can improve both the elongation at break and tensile strength of the stretchable paper.

TABLE 26 Composition and heat treatment of the four sets of handsheets Set # Composition Heat treatment 1 80% treated NBSK and 20% PET/PE bico — 2 80% treated NBSK and 20% PET/PE bico 5 minutes at 160° C. 3 69.4% treated NBSK, 17.4% PET/PE bico — and 13.2% Acronal LA 471S 4 69.5% treated NBSK, 17.5% PET/PE bico 5 minutes at 160° C. and 13.0% Acronal LA 471S

TABLE 27 Physical and mechanical properties of the different sets of handsheets of Example 14 Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 58.2 330 3.11 2.9 67 28 2 65.8 352 10.7 12.4 1010 76 3 75.0 299 16.7 6.2 873 32 4 74.1 328 24.0 15.7 2756 41

EXAMPLE XIV

This example illustrates that the stretchable paper as encompassed herein can be made from blends of different types of pulp. The pulps used for this example were a Northern Bleached Softwood Kraft (NBSK) pulp treated mechanically in a plug-screw feeder at 3.5 bars and a Northern Bleached Harwood Kraft (NBHK) pulp treated under the same conditions. For this example, fourteen different sets of handsheets were prepared from different mixtures of the two pulps and with different amounts of latex added by impregnation. Target grammage prior to impregnation with latex was 60 g/m² in all cases. The composition and mechanical properties of the different sets of handsheets are given in Tables 28 and 29 below.

Results from Table 29 indicate that elongation at break increases with the content of treated NBSK in the pulp mixture and with the latex content in the handsheet after impregnation. Substitution of some NBSK with NBHK is expected to improve sheet formation on commercial paper machines.

TABLE 28 Composition of the fourteen sets of handsheets Set # Furnish Additives 1  100% treated NBSK — 2  5.0% Acronal LA 471S 3 10.0% Acronal LA 471S 4 21.3% Acronal LA 471S 5 66.6% treated NBSK — 6 33.3% treated NBHK 11.0% Acronal LA 471S 7 21.0% Acronal LA 471S 8 33.3% treated NBSK — 9 66.6% treated NBHK 11.3% Acronal LA 471S 10 21.1% Acronal LA 471S 11  100% treated NBHK — 12  5.4% Acronal LA 471S 13 10.1% Acronal LA 471S 14 20.9% Acronal LA 471S

TABLE 29 Physical and mechanical properties of the different sets of handsheets Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 57.64 130 1.88 13.1 185 218 2 60.16 167 8.56 8.8 654 99 3 67.21 162 16.44 10.8 1522 90 4 77.90 165 37.35 12.4 3807 62 5 60.6 144 1.72 11.8 149 185 6 70.0 170 11.52 9.9 936 67 7 72.1 163 37.94 11.6 3564 67 8 59.4 144 1.49 11.0 117 176 9 69.3 167 13.85 9.2 1075 67 10 73.0 163 32.52 10.1 2704 55 11 61.1 149 1.27 10.5 96 182 12 59.9 151 5.66 6.8 326 77 13 63.6 154 11.00 7.8 737 71 14 73.9 154 28.18 8.4 1966 53

EXAMPLE XV

This example illustrates how additives can be added to the pulp prior to mechanical treatment to improve the final properties of the stretchable paper as encompassed herein. The starting pulp used for this example was a Northern Bleached Softwood Kraft (NBSK) pulp at an initial consistency of 47%. The additive was glycerol, added to the pulp at a dosage of 5 wt %. Samples of NBSK with or without glycerol addition were treated mechanically in the plug-screw of a commercial meat grinder (Weston ProSeries tm #8) to induce fiber deformation and curl. FQA measurements confirmed that the curl index obtained with the meat grinder was close to what is obtained with a pilot scale plug screw at a pressure of 3.5 bars. Handsheets were then prepared from the mechanically treated pulps at a target grammage of 60 g/m². Some of the handsheets were finally impregnated with latex. In total, four different sets of handsheets were prepared for this example. Their composition and mechanical properties are given in Tables 30 and 31 below.

Results from Table 31 indicate that handsheets prepared with the pulp containing glycerol prior to mechanical treatment have higher strength, elongation at break, tensile energy absorption and elastic modulus than the corresponding handsheets made from the pulp without glycerol. The same conclusion holds after these handsheets have been impregnated with latex.

TABLE 30 Composition of the four sets of handsheets Set # Pulp Latex addition 1 NBSK, mechanically treated — 2 NBSK, mechanically treated 19.2% Acronal LA 471S 3 NBSK added with glycerol, — mechanically treated 4 NBSK added with glycerol, 19.3% Acronal LA 471S mechanically treated

TABLE 31 Physical and mechanical properties of the different sets of handsheets of Example 16 Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 57.6 172 1.55 12.1 138 209 2 60.2 191 32.59 9.9 2652 69 3 67.2 164 1.65 16.7 200 271 4 77.9 171 37.38 11.6 3396 81

EXAMPLE XVI

These examples illustrate how the properties of the stretchable paper as described herein change with the type of polymer added to the basesheet. The pulp used for this example was a Northern Bleached Softwood Kraft (NBSK) pulp treated mechanically in a plug-screw feeder at 3.5 bars. For this example, five different sets of basesheets were prepared from that pulp at a target grammage of 60 g/m² prior to impregnation. These basesheets were then impregnated with the following polymers: the acrylic copolymer latex Acronal LA 471S used in previous examples, two different grades of PLA latexes manufactured by the company Konishiyasu (grades 1005 and 3000), a medium-chain-length polyhydroxyalkanoate (mcl-PHA) synthesized by Prof. Bruce Ramsay at Queen's University and a mixture of polyvinyl alcohol (PVOH) and sodium borate. The mcl-PHA polymer was first melted at 150° C. prior to its addition to the basesheet. The handsheets impregnated with the PLA latex grade 1005 were subsequently subjected to a heat treatment in an oven at 170° C. for 5 minutes to fully develop the properties of the polymer. The composition and mechanical properties of the different sets of handsheets produced for this example are given in Tables 32 and 33 below.

Results from Table 33 indicate that a wide range of mechanical properties can be obtained by varying the type of polymer added to the basesheet. For example, handsheets impregnated with the synthetic polymer Acronal LA 471S had very high elongation at break. By contrast, handsheets impregnated with the two PLA latexes had high tensile strength and elongations at break sufficient for many applications.

TABLE 32 Composition of the five sets of handsheets Set # Pulp Polymer 1 NBSK mechanically 21.3% Acronal LA 471S 2 treated at 3.5 bars 24.2% PLA (grade 1005) 3 24.0% PLA (grade 3000) 4 58.7% mcl-PHA 5 23.5% PVOH and sodium borate

TABLE 33 Physical and mechanical properties of the different sets of handsheets Tensile TEA Elastic Set Grammage Caliper Stretch Index Index Modulus # (g/m²) (μm) (%) (N · m/g) (mJ/g) (km) 1 77.9 164 37.35 12.4 3810 62 2 82.4 220 7.21 39.6 2050 257 3 79.6 173 9.85 24.3 1750 153 4 161.5 — 13.87 12.0 1335 49 5 83.4 — 3.46 40.6 1020 315

While this disclosure has been described in connection with specific embodiments thereof, it will be understood that it is capable of further modifications and this application is intended to cover any variations, uses, or adaptations, including such departures from the present disclosure as come within known or customary practice within the art, and as may be applied to the essential features hereinbefore set forth, and as follows in the scope of the appended claims. 

1. A method for manufacturing an extensible paper, the method comprising the steps of: providing pulp fibers in form of a pulp having a first length-weighted average curl index (C_(LW1)) between 0 and 0.3; mechanically treating the pulp fibers using a pulp compression process under a pressure of from 2 to 20 bars and expending an energy of 60 to 100 kWh/tonne, to induce fiber deformations such that the resulting pulp fibers have a second length-weighted average curl index (C_(LW2)) higher than the first length-weighted average curl index and between 0.1 and 0.45; forming a wet web using the mechanically treated pulp fibers; drying the wet web under restraint to form a dried web; and adding a polymer to the dried web.
 2. The method of claim 1, wherein the pulp has a consistency above 20% by dry weight before the pulp compression process.
 3. The method of claim 1 or 2, wherein the pressure of the pulp compression process is from 3 to 8 bars.
 4. The method of any one of claims 1 to 3, wherein the pulp fibers are refined prior to mechanically treating.
 5. The method of any of claims 1 to 3, wherein a chemical additive is added to the pulp prior or after mechanically treating said pulp.
 6. The method of claim 5, wherein the chemical additive is a plasticizer.
 7. The method of claim 5, wherein the chemical additive is a strength additive, a sizing agent, or a cross linking agent.
 8. The method of claim 7, wherein the chemical additive is a starch, a polyacrylamide derivative, cellulose fibrils, polyamide-epichlorohydrin, melamine, urea formaldehyde, or a polyimine.
 9. The method of any one of claims 1 to 8, wherein the pulp compression process is a plug screw feeding process.
 10. The method of any one of claims 1 to 8, wherein the pulp compression process is a hammer mill, a Kollergang mill, a BIVIS device, an explosion pulping device or a revolving drum.
 11. The method of any one of claims 1 to 9, further comprising heating and/or chemically treating the pulp after mechanically treating said pulp.
 12. The method of any one of claims 1 to 11, wherein the second length-weighted average curl index (C_(LW2)) is between 0.25 and 0.45.
 13. The method of any one of claims 1 to 12, wherein the polymer is a bio-based polymer, a biodegradable polymer and/or a petroleum-based polymer.
 14. The method of claim 13, wherein the petroleum-based polymer is a thermoplastic polymer or copolymer selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, acrylic polymer, acrylic copolymer, vinyl acetate polymers, vinyl acetate copolymers, and vinyl chloride-vinylidene chloride copolymers.
 15. The method of claim 13, wherein the polymer is in the form of an aqueous dispersion or latex.
 16. The method of any one of claims 1 to 15, wherein the polymer is added by impregnation.
 17. The method of claim 17, wherein impregnation of the dried web comprises a web immersion in a polymer bath followed by removal of the excess polymer and a further restrained drying.
 18. The method of any one of claims 1 to 17, wherein the polymer is added in an amount of from 2 to 40 wt %.
 19. The method of any one of claims 1 to 18, wherein a cross-linking agent is added to the pulp after mechanically treating the pulp fibers.
 20. The method of any one of claims 1 to 19, wherein the pulp is a mechanical, chemi-mechanical or chemical pulp.
 21. The method of any one of claims 1 to 20, wherein the pulp is a combination of different pulps.
 22. An extensible paper comprising: pulp fibers having a length-weighted average curl index C_(m) of between 0.1 and 0.45; and a polymer in an amount of from 2 to 40 wt % based on the weight of the extensible paper; wherein the extensible paper has a Gurley air resistance below 20 s/100 mL and an elongation at break of at least 7%.
 23. The extensible paper of claim 22, wherein the elongation at break is from 10% to 90%.
 24. The extensible paper of claim 22 or 23, wherein the extensible paper is a polymer-impregnated extensible paper.
 25. The extensible paper of any one of claims 22 to 24, wherein the curl index of the fibers contained in the extensible paper is of from 0.25 to 0.45.
 26. The extensible paper of claim 24, wherein the impregnated polymer is present in an amount of 10 wt %.
 27. The extensible paper of any one of claims 22 to 26, wherein the polymer is a bio-based polymer, a biodegradable polymer and/or a thermoplastic polymer or copolymer.
 28. The extensible paper of claim 27, wherein thermoplastic polymer or copolymer selected from the group consisting of polyethylene, polypropylene, polyethylene terephthalate, styrene-butadiene copolymers, acrylonitrile-butadiene copolymers, acrylic polymer, acrylic copolymer, vinyl acetate polymers, vinyl acetate copolymers, and vinyl chloride-vinylidene chloride copolymers.
 29. The extensible paper of any one of claims 22 to 29, wherein the polymer is a latex.
 30. An extensible paper produced by the method of any one of claims 1-21. 